1. Field of the Invention
The present invention relates to a liquid crystal composition, a selectively reflective film showing selective reflectivity for a range of light from ultraviolet to infrared and a liquid crystal color filter.
2. Description of the Related Art
In recent years, attention has been focused on liquid crystal materials such as a cholesteric liquid crystal with a helical structure, which display a variety of selectively reflected color lights depending on a twisting power (a twist angle) of a helix. Moreover, because these liquid crystal materials have excellent selective reflectivity and color purity of selectively reflected light, these materials have been widely used in applications such as an optical film, a liquid crystal color filter and a recording medium.
A color filter used in a color liquid crystal display or the like, for example, is generally composed of pixels for each of red (R), green (G) and blue (B), and a black matrix is formed in gaps between adjacent pixels for the purpose of increasing display contrast. This kind of color filter has mainly been made of resin in which pigment is dispersed or resin dyed with dye. Fabrication processes therefor are generally: applying colored resin liquid on a glass substrate using a spin coating method or the like to form a colored resist layer and patterning the resist layer through a photolithographic method into color filter pixels; or printing color pixels directly on a substrate.
However, in the fabrication process using printing, for example, a problem arises that the pixel resolution is low, making it difficult to render the process correspond to forming a very fine image pattern. In the fabrication process using the spin coating method, a problem arises that a great loss of material is caused and significant coating nonuniformity occurs in a case of applying on a substrate having a large area. In a fabrication process using electrodeposition, on the other hand, a color filter having a relative high resolution and a more uniform colored layer can be obtained, but this fabrication process is complex and liquid management therein is also difficult.
Considering the above problems and difficulties in the prior art, there has been a need for a fabrication process for a color filter capable of producing a high quality color filter with a high efficiency, simplicity, and convenience, and with less loss of material.
High transmittance and color purity have been required and efforts to meet these requirements for color filter performance have been made in recent years by optimizing the kind of dye and the dyed resin in the processes employing dyes, and by using a more finely dispersed pigment in the processes employing pigments. Extremely high requirements, however, have been imposed on transmittance and color purity of a color filter used in recent liquid crystal display (LCD) panels. Especially in a reflective LCD color filter, on one hand, it is difficult to fulfill requirements for a paper-white white display and contrast, and for color reproducibility. While on the other hand, any color filter obtained by prior art fabricating processes, such as dying resin with a dye or dispersing a pigment, is of a light absorbing type, thus improvement in color purity through further increasing transmittance has almost reached its limit.
To solve these conditions, a polarization color filter made with cholesteric liquid crystal is known. The polarization color filter reflects a predetermined light quantity and transmits the rest to display an image, and is therefore high in utilization efficiency of light, and shows performance superior to that of a light absorbing type color filter in view of transmittance and color purity. On the other hand, a process has been generally adopted for a fabrication process in which a film is formed on a substrate using a spin coating method or the like since a uniform thickness is ensured with the spin coating method. However, these methods have a cost disadvantage from a great loss of materials.
A useful process has been implemented that uses a photoreactive type chiral compound, as means to ensure uniformity of color purity and the like of a color filter film and further to reduce the number of steps of a fabrication process. The process uses a principle that, when a liquid crystal composition including a photoreactive chiral compound is patternwise irradiated with light of wavelengths to which the chiral compound is photosensitive, a reaction of the photoreactive chiral compound progresses according to an intensity of the irradiation energy, thereby altering helical pitches (a twist angle of a helix) of the liquid crystal compound. Thus, selectively reflected colors are produced per pixel by only pattern exposing with different quantities of light. That is, an advantage can be obtained in that patterning in color filter formation is completed with a single mask exposure using a photomask which allows different degrees of illuminance.
Accordingly, after imagewise patterning with light irradiation, the patterned cholesteric liquid crystal compound is fixed, to thereby allow formation of a film functioning as a color filter. This technique can also be applied in an optical film, image recording medium and the like.
However, usually, a number average molecular weight of a photoreactive chiral compound has a value of approximately from 300 to 800, and if a photoreactive chiral compound having one of these molecular weights is adopted, an isomerized photoreactive chiral compound might be moved by molecular diffusion some time after the patterning and before polymerization. Therefore, a problem occurs that, if the photoreactive chiral compound has moved and is then polymerized and fixed, the produced color filter suffers from reduced resolution and forms a foggy image.
The present invention has been made in view of the prior art problems. Therefore, it is an object of the present invention to provide a selectively reflective film exhibiting high resolution, a liquid crystal color filter including the film and a liquid crystal composition capable of forming the film and the filter.
The present invention has been accomplished on the basis of a finding that a photoreactive chiral compound having a larger molecular weight is inhibited from being moved through molecular diffusion. Specifically, the present invention provides the following:
 less than 1 greater than  A liquid crystal composition comprising: a liquid crystal compound having at least one polymerizable group; a photoreactive chiral compound; and a polymerization initiator; wherein a number average molecular weight of the photoreactive chiral compound is from 1,000 to 30,000.
 less than 2 greater than  A selectively reflective film comprising a liquid crystal composition, wherein the liquid crystal composition includes a liquid crystal compound having at least one polymerizable group; a photoreactive chiral compound having a number average molecular weight of from 1,000 to 30,000; and a polymerization initiator.
 less than 3 greater than  A liquid crystal color filter comprising a selectively reflective film, wherein the selectively reflective film includes a liquid crystal composition containing a liquid crystal compound having at least one polymerizable group; a photoreactive chiral compound having a number average molecular weight of from 1,000 to 30,000; and a polymerization initiator.